The present invention is concerned with a hydraulic brake system for avoiding high brake or traction slip values. Brake systems of this type are disclosed, for example, by DE-OS 36 35 054. According to FIG. 1 of this Offenlegungsschrift, the brake system comprises a tandem master cylinder connected to the wheel brakes through branched brake conduits. Provided in each of the branch conduits is an inlet valve which is actuated electromagnetically. Moreover, each of the wheel brakes is in communication with a pressure fluid reservoir through a return conduit provided with an outlet valve. The outlet valve likewise is electromagnetically actuated. Each of the working chambers of the master cylinder is in communication with a pump. With the aid of this arrangement, the wheels can be prevented, during deceleration, from locking and an excessive brake slip from occurring. For this purpose, the inlet and outlet valves are actuated. Once the inlet valve is opened, pressure fluid flows from the pump into the wheel brake to thereby increase the brake pressure. Once the outlet valve is opened, pressure fluid will flow from the wheel brake into the pressure fluid reservoir to thereby cause the pressure in the wheel brake to decrease. By successively actuating the inlet and outlet valves, a pressure is adjustable in the wheel brakes that is in harmony with the forces transferred between tire and roadway to insure that the wheel has an optimum slip value.
It is known that anti-locking brake systems of this type also permit controlling traction slip. For that purpose, a separator valve is inserted into the branch conduit of the brake conduit leading to the wheel brake of the actuated wheel. The pressure fluid source, composed of pump and accumulator, is connected, through an intake valve, to the branch conduit between the separator valve and the inlet valve. In a control of the traction slip, in which racing of the wheels is to be avoided, the wheel brakes of the actuated wheels are connected directly to the pressure fluid source, with the separator valve preventing the pressure fluid from flowing back into the master brake cylinder. The pressure control is through actuation of the inlet and outlet valves, as it is in a control of brake slip.
Circuit configurations of this type have a number of disadvantages.
The inlet valves are provided with diaphragms which determine the rate of pressure increase. Throttlings of this type will be necessary to have a positive impact on the control pattern. The optimum throttling rates for a traction slip control and a brake slip control, respectively, vary. As the inlet valve is operative in both types of control, the selected size of the diaphragm is but a compromise which, in either case, results in a deterioration of the control quality.
Moreover, the brake conduit contains two valves, namely the inlet valve and the separator valve. The separator valve also causes an unavoidable throttling effect with the result that the wheel brakes, in the event of a rapid actuation of the brake pedal, respond only with a slowed-down or delayed effect.